After thinking it over, I wonder if it somehow ties into how Devil Fruits are created?
Devil Fruits are, according to Vegapunk, different visions for the future.
(Chapter 1069)
So, what if Conqueror's is actually a part of this? What if Conqueror's is basically you realizing what kind of future YOU want? A goal to strive for?
ALL Haki is about a person's amount of willpower. Their determination to achieve short-term goals. But, Conqueror's Haki is specifically about having a long-term goal, and trying to make it into reality. Conqueror's is about having the will to change the world around you. Imposing YOUR vision for the future onto the rest of the world. Thinking ahead more than Zoro usually does. The difference between regular Haki and Conqueror's could just be having longer-term goals in mind.
Conqueror's Haki could be basically the "seed" that allows a Devil Fruit to eventually be "grown".
That might explain why Zoro thought only Captains could have Conqueror's. Captains are the ones who decide where the rest of the crew goes. They're the ones who decide what they do. The Captains are the ones who set the goals for the entire crew. But, Zoro just realized that members of the crew can also have their OWN goals to reach, too. The future that the Captain wants, doesn't need to be incompatible with the future that the members of the crew want. Which seems like a relatively simple idea, but it may have just clicked into place for Zoro. Zoro may have JUST been able to "envision his future".
It's still imposing your will onto others. But, it's slightly different than that, too. It's about imposing your will onto EVERYTHING. Including the future, and finding a way to mold the future into what you desire.
Zoro's always been working towards his goal of being the World's Strongest Swordsman. Which is why he's had Conqueror's flowing out of him without him even realizing it. But, it's possible that it's not until this moment that he's gotten a "glimpse" of that future in his mind's eye. Zoro THINKING about his future allowed him to unlock his potential. Because...Zoro doesn't really think that much. He just DOES things. Zoro kind of had the opposite problem of Sanji if that's the case. Because Sanji probably has a tendency to OVERTHINK things. Zoro thinks too little, and Sanji thinks too much, and both of those got in the way of them unlocking Conqueror's earlier.
I don't know, I'm still chewing this over. And it might be beneficial to see a couple more different translations and interpretations of what this could mean.